Incorporation, alloy electrodeposition

Hundreds of baths exist for electrodeposition of gold and its alloys . The latter are more wear resistant, so better for contacts . Polymers incorporated in cyanide-bath deposits affect wear and contact resistance . 

The incorporation of a third element, e.g. Cu, in electroless Ni-P coatings has been shown to improve thermal stability and other properties of these coatings [99]. Chassaing et al. [100] carried out an electrochemical study of electroless deposition of Ni-Cu-P alloys (55-65 wt% Ni, 25-35 wt% Cu, 7-10 wt% P). As mentioned earlier, pure Cu surfaces do not catalyze the oxidation of hypophosphite. They observed interactions between the anodic and cathodic processes both reactions exhibited faster kinetics in the full electroless solutions than their respective half cell environments (mixed potential theory model is apparently inapplicable). The mechanism responsible for this enhancement has not been established, however. It is possible that an adsorbed species related to hypophosphite mediates electron transfer between the surface and Ni2+ and Cu2+, rather in the manner that halide ions facilitate electron transfer in other systems, e.g., as has been recently demonstrated in the case of In electrodeposition from solutions containing Cl [101]. 

The mechanism of co-deposition of boron in Ni-B coatings prepared by electrodeposition technique is not yet fiilly understood. It is assumed that boron is incorporated into Ni-B coating due to the adsorption of DMAB on the surface of already formed nickel which is then decomposed to elemental boron. Therefore, the amount of boron codeposited with nickel can be determined by the distribution of DMAB, and the thickness of the diffusion layer at the cathode surface, regardless of the electrode potential [26]. The mechanism of incorporation of zinc into electrodeposited Ni-B matrix can be explained on the basis of formation and subsequent reduction of ZuNi-t-aj species at the surface of the steel substrate. Miranda et al. have observed that the formation of Ni-rich phase in Zn-Ni alloy occurs at low potentials through the formation and subsequent reduction of ZnNi ad species. This observation tends to suggest that a similar phenomenon occurs in the electrodeposition of Ni-Zn-B which is very much similar to Ni-Zn-P coatings[27]. 

Nickel contents of up to about 15% are incorporated in a zinc coating, usually by electrodeposition but also by mechanical coating. (The addition of 0.06-0.11% nickel in hot dip galvanizing, to suppress excessive Zn alloy formation, does not influence the corrosion resistance Van Eijnsbergen unpublished report). 

Sulfite electrolytes for colored gold alloys are also in use for decorative deposition thanks to their excellent distribntion and leveling properties and also for their superior ductility compared to gold alloy deposits from acid cyanide electrolytes giving similar shades and colors [90], Alloy plating from snlflte electrolytes comprises binary and ternary compositions, such as Au-Cu, Au-Pd, and An-Cn-Pd—already reported by Rapson [3] and later incorporated into proprietary processes [91-93], The later is an alloy finish widely used in the electroplating of spectacle frames. Example formnlations of nentral and alkaline sulflte baths for the electrodeposition of gold alloy deposits are reported in Table 11.8. 

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